http://llvm.org/bugs/show_bug.cgi?id=8959
Summary: inline asm "0" constraint doesn't implicitly cast
Product: new-bugs
Version: trunk
Platform: PC
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
AssignedTo: [email protected]
ReportedBy: [email protected]
CC: [email protected], [email protected]
This is a breakout from bug 3373 comment 11. New testcase:
int test_and_set(volatile int *addr)
{
unsigned char oldval;
/* Note: the "xchg" instruction does not need a "lock" prefix */
__asm__ __volatile__("xchgb %0, %1"
: "=Q"(oldval), "=m"(*addr)
: "0"(0xff), "m"(*addr) : "memory");
return (int)oldval;
}
which produces:
$ clang t.c -S -o -
t.c:7:25: error: unsupported inline asm: input with type 'int' matching
output
with type 'unsigned char'
: "0"(0xff), "m"(*addr) : "memory");
^~~~
1 error generated.
Ian Taylor explains: "Clang is incorrectly assuming that when given an operand
number as a constraint that the type of a constant must be identical to the
type of the other operand. When the operand is a constant, it should be
converted to the relevant type as though in an assignment. When the operand is
a variable of some sort, it must of course have exactly the correct type."
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